This is has to do with the layers of a movie. By default, only the top layer is displayed, hiding the pictures behind it. You will need to use blank frames to hide the image on top and display an image underneath the top image.
If you have several objects that sit atop one another, and you want the objects in front to disappear during your movie to reveal an object in the back, you will need to use blank spaces to cover the front objects. The blank space is located in the cutting room in the bottom left corner of the screen. It appears as a piece of film. When you add a blank space to the timeline, that object will not appear for the length of the blank space, allowing objects behind the blank space to appear. You can always make your blank spaces bigger or smaller.